About the pawns above your castled king

Hey guys. I was just wondering something. I don't know if this is chess advice or what it is, I got it from somewhere but I need some help from someone who knows what they're doing when they are applying "chess opening" principles. I've seen the "everything you need to know" video series for beginners, at least the first two parts (there are five) and someone correct me if I'm wrong but........is it true that you should be careful when considering moving your pawns forward? - the ones that are above the already castled king? I would think to keep them there would be a good idea so that the king is safe guarded. After all, if you keep them there, then you can block attacks to your king! But just how much of this is a STRICT rule?? Because I find that if I can move those pawns, I can develop quite a nice pawn structure. If someone could point me in the right direction I would be most grateful. My online chess rating as of this writing is only 841 lol But you know......you never know how high it's possible for that to go if I can just get an answer to this question. At least by someone who knows this rule and how strict it is?

Anyway, if it is a rule, I assume the answer to how strict it is could take a long while so......to whoever responds, just give me a few hints. Ok, bye

Thanks